I might could talk Southern

One of the groups I sang with Saturday night was a country and bluegrass group whose leader jokingly told me, “You better speak Southern if you want to sing with us.” I told him, “I might could do that,” in my best Appalachian twang, and got in. Today my Tennessean officemate unconsciously used the same construction, so I started wondering where it came from.

The use of so-called “double modal” constructions is quite common in the
South and Southwest. I come from Dallas originally, and such
constructions as you have cited are common there in everyday speech, and
they serve a real linguistic purpose: modal forms such as ‘could’ and
‘should’ are ambiguous in Modern English, as they have both an
indicative and a subjunctive sense. For example, “I could come” can mean
either “I was able to come” (past indicative of ‘can’) or “I would be
able to come” (subjunctive). In German, the two forms are distinct:
“ich konnte kommen” vs. “ich koennte kommen”. The use of double modal
constructions with ‘may’ or ‘might’ serves to reintroduce this
distinction. Thus, for a Southerner, “I might could come” or “I may
could come” carry the subjunctive meaning, whereas “I could come” is
only indicative in meaning….

The use of double
modals in Southern American English fills a gap in Standard English
grammar, namely the loss of inflectional distinction in English between
indicative and subjunctive modals. Dialect or regional forms are often
more progressive in gap-filling than is a standard language. Consider
the sad case of ‘you’, which is ambiguous in Standard English between
singular and plural meanings. Here the regional forms have been quite
productive: “y’all” in the South (***only plural!!!!***) or similar
forms elsewhere.